A number of crystalline metal alloy materials are generally brittle and hard to machine. Examples of such materials are metal alloys which are permanent magnets. Because of the hardness and brittleness of these materials, the machining of these materials into small magnets which can be fitted into magnetic circuits is very expensive. therefore, a specific need exists for a more economical method of forming magnetic materials into a desired shape.
The instant invention cures this problem by preparing a bulk amorphous metal alloy article which is easily machinable and which can be recrystallized to give the appropriate magnetic material. Amorphous materials are well known in the art and are produced in a number of ways. One such method disclosed in U.S. Pat. Nos. 4,537,624 and 4,537,625 involves the thermal or chemical decomposition of a precursor compound to give an amorphous material. Another method of forming an amorphous material is through the rapid quenching (10.sup.6 .degree. C./second) of a molten material. See, for example, U.S. Pat. No. 4,594,104 and references therein. A further method of producing an amorphous material is through mechanical alloying. C. C. Koch et al. Appl. Phys. Lett, 43, 1017 (1983). Mechanical alloying is a physical process which takes place during high energy ball milling. More specifically, mechanical alloying is characterized by the repeated welding, fracturing and rewelding of powder particles. Mechanical Alloying can produce an amorphous powder. For a more detailed explanation see P. S. Gilman and J. S. Benjamin, "Mechanical Alloying", Ann. Rev. Mater. Sci., Vol. 13, 279-300 (1983).
These amorphous materials usually cannot be formed into a bulk amorphous metal alloy article by ordinary forming methods (i.e. ordinary with respect to crystalline material) such as cold or hot-pressing. By bulk or consolidated article is meant an article which has high strength (not easily deformed), integrity, hardness, etc. However, some of these amorphous materials can be formed into a bulk amorphous metal alloy article by more severe methods such as high speed compaction. For a description of the high speed compaction process, see U.S. Pat. No. 4,594,104. Examples which fall into the latter category include NiTi, SmCo, NiZr, NiHf, and CuZr.
The prior art additionally discloses two specialized methods of forming bulk amorphous metal alloy articles. First, U.S. Pat. No. 4,557,766 discloses that an intimate mixture (which is crystalline) of the components of the metal alloy is formed by chemically reducing compounds of the desired components. The intimate mixture can be formed into a bulk amorphous metal alloy article by standard methods such as hot-pressing and then heated to induce the amorphous state. Second, U.S. Pat. No. 4,640,816 discloses a method of cold-working the precursors into a thin sheet or film and then heating the sheet to induce the amorphous state. It is also necessary that one of the precursors be in the form of a film, foil, sheet, etc. Thus, this method gives very limited shapes of the amorphous metal alloy.
In marked contrast to the prior art, the present invention provides a simple method of forming a bulk amorphous metal alloy article from at least one matrix metal and at least one element which is a fast diffuser in the matrix meal (hereinafter fast diffuser element). Unlike the method described in U.S. Pat. No. 4,594,104, which uses an elaborate system to precipitate an intimate mixture of the components of the metal alloy, the instant invention starts with simple crystalline powders or flakes of the desired components which are mechanically alloyed into a substantially amorphous intimate powder mixture. By substantially amorphous is meant that the powder mixture is at least 50% but less than 100% amorphous.
The prior art does not disclose nor suggest that it would be advantageous to mechanically alloy a matrix metal and a crystalline diffuser element to a point that some degree of crystallinity remains. It is applicant who has surprisingly discovered that the presence of a crystalline component in a mechanically alloyed powder mixture allows one to form bulk amorphous metal alloy articles using standard forming methods such as cold-rolling or hot-pressing. Applicant has also discovered, absent any teaching in the prior art, that in order to form bulk amorphous metal alloy articles, each particle of the powder mixture must have a modulated structure. By modulated structure is meant a concentric gradient of the respective components of the particle.
In summary, this invention provides a simple method of forming a bulk amorphous metal alloy article by routine methods such as cold or hot-pressing. This presents a ignificant advance in the art by forming a bulk amorphous metal alloy article without resorting to such costly and limited (i.e. shape limited) methods as high speed compaction.